Document v19 migration mechanics and disk-space requirements #809

Problem

The v19 README gives users a migration command without explaining what the migration does, which phases may take time, what is written, or how much temporary disk space to reserve. Operators cannot assess risk or capacity before approving the migration.

Scope

• Explain discovery, inventory, rewrite, scratch verification, finalize, and verification phases in operator language.
• State which refs and Git objects are read, created, or compare-and-swap updated.
• Explain dry-run versus apply behavior and the exact point after confirmation where writes begin.
• Explain scratch repository placement/lifetime, cleanup, interruption behavior, and recovery guarantees.
• State whether retained data may be temporarily duplicated and provide a conservative disk-space estimate/formula from repository evidence.
• Add a preflight display of estimated scratch/free-space requirements where the implementation can measure them reliably.
• Explain how multiple graphs in one repository affect scope and estimates.

Acceptance

Before confirming, an operator can understand the migration phases, expected duration drivers, mutation/atomicity boundary, recovery posture, and how much free disk space to set aside for the selected graph.